Various types of container covers in shapes such as crowns or caps, furnished with liners or packings such as various rubbers, resins, paper or cork have been widely used in the past. Caps having liners comprising low density polyethylene and vinyl chloride resin have been gradually coming into widespread use since such liners are easily formed, have superior sealability, protect the flavor of the contained beverages, and have low production costs.
One method of manufacturing caps having liners is disclosed in Japanese patent application Publication No. 41-5588 (1966) and comprises the step of supplying a piece of molten thermoplastic resin to the inner surface of a cap shell and the step of pressing the piece between the shell body and a cooled press to form the liner shape. A further method is disclosed in Japanese patent application Publication No. 48-5706 (1973) where the thermoplastic resin is supplied as a prepared form to the inner surface of the cap shell after which the form is softened by heating the shell. The form is then pressed between the cap shell and a cooled press to form the liner shape.
It is necessary that the liner of a cap engage with a container mouth in a tightly sealed manner, and consequently the outer periphery of the liner that engages with the container mouth should be furnished with one or more ring-shaped projections or with a concave channel.
Prior methods of press forming a liner have the advantage of simultaneously forming the liner shape from the piece of thermoplastic resin and sealing the liner to the container cover shell. However, the sealing performance of press formed liners has in many instances been inferior to that of prepared form disc-shaped liners punched from liner sheets. This is due in part to such factors as the compressive force applied to the liner, contact with the contained product or the vapor of the contained product and the temperatures applied during sterilization, all of which may cause microcracks to form in the liner and particularly on the peripheral portion of the liner which is the most important portion from a sealing function. The tendency for formation of this sort of cracking increases as the compression forming of the liner is carried out at greater speed and as annular projections and concave channels are formed for more effective dimensioning of the liner periphery.
Thus, caps furnished with press formed resin liners, in spite of the fact that the resin itself has an essentially superior performance, have not in all instances shown satisfactory sealability, liner anti-peelability or long-term sealability after heat treatment.
It is therefore an object of the present invention to provide for a cap having a press formed resin liner and a method of manufacturing the same where the liner has superior stress cracking resistance properties, under environmental conditions, under compressive forces received by the resin liner during sealing, under contact with the contents of containers or their vapors and under high temperature applied to the contents during sterilization.
A further object of the present invention is to provide for a press formed resin lined cap and a method of manufacturing the same where the cap has a superior combination of sealability, liner peeling resistance and long-term sealability after heat treatment than prior art caps.
Still another object of the present invention is to provide for a resin lined cap and a method of manufacturing the same where stress cracking is effectively prevented in caps having ring-shaped projections and/or concave channels which are formed at the outer periphery of the liner where it contacts a container opening.
Yet another object of the present invention is to provide for a resin lined cap where the diameter of the cap is comparatively large and where a piece of thermoplastic resin used as a liner is applied inside the cap shell in a comparatively small amount, and where stress cracking is effectively prevented even in cases when the piece of resin is pressed and expanded to a comparatively large diameter.
A further object of the present invention is to provide for a resin lined cap where stress cracking is prevented even in cases where the liner shape is formed from a piece of resin at high speeds.